Question

A 40.0 mL solution containing 0.500 g of KHP was titrated with NaOH solution of unknown concentration, and the pH of the solution was measured after each known amount of NaOH was added. (KHP=potassium hydrogen phthalate; formula=KHC₈H₄O₄; molar mass=204.22 g/mol). The acid base reaction occurs according to the following net ionic equation:

HC₈H₄O₄^- (aq) + OH^- (aq) ®C₈H₄O₄^2- (aq) + H2O

1. What is the molar concentration of KHP in the solution?
2. If the titration required 24.0 mL of NaOH to reach equivalent point, calculate the molar concentration of NaOH solution and the molar concentration of phthalate ion, C₈H₄O₄^2- at equivalence point.
3. At half-way to the equivalence point, the solution was found to have pH=5.40. What is the k_a of hydrogen phthalate ion, HC₈H₄O₄^-, and the k_b of its conjugate base, C₈H₄O₄^2-?
4. Using the concentration of phthalate ion, C₈H₄O₄^2-, at equivalence point and its k_b value, determine [OH^-] and the pH of the solution at equivalent point. At equivalent point, the conjugate base undergoes a hydrolysis reaction as follows:

C₈H₄O₄^2- (aq) + H₂O ÛHC₈H₄O₄^- (aq) + OH- (aq)

Answer 1